Curious if anyone has a picture of or knows what exactly were behind these rear doors marked "HP TEL"

I know these were put in special locations in the 20s/30s where there were high pressure water lines, I have never seen one with the rear door open and whatever was behind it intact, I'm guessing it just had a telegraph key connected specially to the water pumping facility but I'm just guessing.
My post has an openable rear door but the door doesn't have the HP TEL on it and it only opens up into the back of the inner box and it's bracket.
I'm guessing not a lot of these if any still have whatever was behind that rear door originally- still intact.

I know these were put in special locations in the 20s/30s where there were high pressure water lines, I have never seen one with the rear door open and whatever was behind it intact, I'm guessing it just had a telegraph key connected specially to the water pumping facility but I'm just guessing.
My post has an openable rear door but the door doesn't have the HP TEL on it and it only opens up into the back of the inner box and it's bracket.
I'm guessing not a lot of these if any still have whatever was behind that rear door originally- still intact.